In 2004 a researcher named Joshua Broman-Fulks from the University of Southern Mississippi tested whether exercise would reduce anxiety sensitivity. He found fifty-four college students with generalized anxiety disorder who had elevated anxiety sensitivity scores and who exercised less than once a week. He randomly divided his sedentary subjects into two groups, both of which were assigned six twenty-minute exercise sessions over two weeks. The first group ran on treadmills at an intensity level of 60 to 90 percent of their maximum heart rates. The second group walked on treadmills at a pace of one mile per hour, roughly equal to 50 percent of their maximum heart rates.
Both regimens tended to reduce anxiety sensitivity, but rigorous exercise worked more quickly and effectively. Only the high-intensity group felt less afraid of the physical symptoms of anxiety, and this distinction started to show up after just the second exercise session. The theory is that when we increase our heart rate and breathing in the context of exercise, we learn that these physical signs don’t necessarily lead to an anxiety attack. We become more comfortable with the feeling of our body being aroused, and we don’t automatically assume that the arousal is noxious.
This is a key finding given the notion of anxiety as a cognitive misinterpretation. By using exercise to combat the symptoms of anxiety, you can treat the state, and as your level of fitness improves, you chip away at the trait. Over time, you teach the brain that the symptoms don’t always spell doom and that you can survive; you’re reprogramming the cognitive misinterpretation.
The fact that aerobic exercise works immediately to fend off the state of anxiety has been well established for many, many years. It’s only more recently, however, that researchers have started to pin down how this works.
In the body, physical activity lowers the resting tension of the muscles and thus interrupts the anxiety feedback loop to the brain. If the body is calm, the brain is less prone to worry. Exercise also produces calming chemical changes. As our muscles begin working, the body breaks down fat molecules to fuel them, liberating fatty acids in the bloodstream. These free fatty acids compete with tryptophan, one of the eight essential amino acids, for slots on transport proteins, increasing its concentration in the bloodstream. The tryptophan pushes through the blood-brain barrier to equalize its levels, and once inside, it’s immediately put to use as the building block for our old friend serotonin. In addition to the boost from tryptophan, the higher brain-derived neurotrophic factor (BDNF) levels that come along with exercise also ramp up levels of serotonin, which calms us down and enhances our sense of safety.
Moving the body also triggers the release of gamma-aminobutyric acid (GABA), which is the brain’s major inhibitory neuro transmitter (and the primary target for most of our antianxiety medicines). Having normal levels of GABA is crucial to stopping, at the cellular level, the self-fulfilling prophecy of anxiety — it interrupts the obsessive feedback loop within the brain. And when the heart starts beating hard, its muscle cells produce a molecule called atrial natriuretic peptide (ANP) that puts the brakes on the hyperaroused state. ANP is another tool the body uses to regulate the stress response, which I’ll explain more later.
As for the trait, the majority of studies show that aerobic exercise significantly alleviates symptoms of any anxiety disorder. But exercise also helps the average person reduce normal feelings of anxiousness. One interesting study in 2005 measured the physical and mental effects of exercise in a group of Chilean high school students for nine months. The researchers divided 198 fifteen-year-olds into two groups: the control group continued with a once-a-week, ninety-minute gym class, and the other embarked on a program of its own design, rigorously exercising during three ninety-minute sessions per week throughout the school year. The study was meant to assess general mood changes in a healthy population, but scores relating to anxiousness really stood out on the students’ psychological tests. The experimental group’s anxiety scores dropped 14 percent versus a statistically insignificant 3 percent for the control group (an improvement that could be explained by the placebo effect). Not coincidentally, the experimental group’s fitness levels improved 8.5 percent versus 1.8 percent for the control group. Clearly, there is a connection between how much you exercise and how anxious you feel.

from the book SPARK by John Ratey